Capacitors for use in electronic instruments such as portable telephone and personal computer are demanded to have a small size and a large capacitance. Among these capacitors, a tantalum capacitor is preferred because of its large capacitance for the size and good performance.
Moreover, recent electronic devices are demanded to operate at a low voltage and high frequency with low noise, and also a lower equivalent series resistance (ESR) is required for a solid electrolyte capacitor.
In this tantalum capacitor, a sintered body of tantalum powder is generally used for the anode moiety. This powder is molded and then sintered, whereby the powder is integrated and works out to an electrode called a sintered body. The inside of this sintered body takes a three-dimensional complicated form resulting from the particles of the powder being electrically and mechanically connected with each other. On the surface of this sintered body including the surface of the inside void, a dielectric film layer is formed and thereinto, a material as a counter electrode is impregnated, whereby a capacitor is fabricated. As long as the dielectric film layer uniformly adheres to the inside or outside surface of the sintered body, the capacitance of the capacitor manufactured greatly depends on, microscopically, the contact state of the counter electrode material with the dielectric film layer.
In order to increase the capacitance of the tantalum capacitor, it is necessary to increase the mass of the sintered body or to use a sintered body increased in the surface area by pulverizing the tantalum powder.
The method of increasing the mass of the sintered body necessarily involves enlargement of the capacitor shape and cannot satisfy the requirement for downsizing. On the other hand, in the method of pulverizing tantalum powder to increase the specific surface area, the pore diameter of the tantalum sintered body decreases or closed pores increase at the stage of sintering, as a result, impregnation of the cathode agent at a later step becomes difficult.
For example, assuming that when an aqueous phosphoric acid solution is used as a counter electrode material, the contact state of the solution with a dielectric film layer is complete and the capacitance appearance ratio (also called a cathode agent impregnation ratio) at the time is 100%, a capacitance appearance ratio of 100% can be hardly attained in the case of using a counter electrode material having high viscosity, particularly a solid electrode material. In particular, when the average particle size of tantalum powder is small or the sintered body manufactured from a tantalum powder has a large shape, the difficulty increases and in an extreme case, the capacitance appearance ratio cannot reach even 50%. With such a low capacitance appearance ratio, the capacitor manufactured cannot have a sufficiently high moisture resistance.
In case the tantalum powder to produce a tantalum sintered body has a small pore diameter, it essentially leads to a small pore diameter of the tantalum sintered body and a low capacitance appearance ratio. Consequently, a problem arises that an ESR cannot be lowered.
As one of means for solving these problems, a capacitor using a sintered body as an electrode may be considered, which sintered body is produced using an electrode material providing a dielectric material having a dielectric constant larger than that of tantalum and gives a high capacitance appearance ratio.
As such an electrode material which can be supplied industrially, niobium having a dielectric constant larger than that of tantalum and having a large reserve is known.
JP-A-55-157226 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a method for producing a sintered device for capacitors, where agglomerated valve-acting metal powder having a particle size of 2.0 μm or less is molded under pressure and then sintered, the molded and sintered body is cut into fine pieces, a lead part is joined therewith and these are again sintered. However, the details on the producing method and properties of the niobium capacitor are not described in this patent publication.
U.S. Pat. No. 4,084,965 discloses a capacitor using a sintered body of niobium powder having an average particle size of 5.1 μm obtained by hydrogenating and pulverizing a niobium ingot. However, the capacitor disclosed has a large leakage current (hereinafter sometimes simply referred to as “LC”) value and is of little practical use.
JP-A-10-242004 (WO 98/38660) discloses a technique of partially nitriding a niobium powder and thereby improving the LC value.
The tapping density of a niobium powder for a capacitor is an important factor in molding operation of the niobium powder. The tapping density of conventional niobium powder is 2.5 g/ml or more, specifically about 4 g/ml, and this is insufficient for the molding.
That is, if such a niobium powder is molded and sintered to prepare a sintered body, the niobium powder poorly flows from the hopper of a molding machine to the metal mold and it is difficult to weigh a constant amount of niobium powder and flow it into the metal mold. As a result, it causes problems such that the shape of the molded article is not satisfactorily stabilized, the molded article and the sintered body are deficient in the strength, and a capacitor having bad LC is produced frequently. If a special molding apparatus capable of also handling a powder having bad flowability is used, the molding cost excessively increases and this is not practical.
As such, conventionally known niobium powder for capacitors has a problem that the powder cannot be fully adapted to continuous molding and the productivity of capacitors is low.